Optical communications systems employing optical fibers have reached a state of technical sophistication and increasing commercial importance. Such systems have greatly increased the capacity of present and contemplated telecommunications systems. Nonetheless there is a relentless demand for ever increasing capacity.
Wavelength division multiplexed systems (WDM systems) offer the prospect of greatly increased capacity, and bi-directional WDM systems offer more efficient use of optical fiber networks and higher transmission capacity than unidirectional systems.
Amplifiers are particularly important in bi-directional optical communications systems. In such systems, signal crosstalk due to Rayleigh backscattering can degrade transmission performance. And in some systems using rare earth doped amplifiers, amplified spontaneous emission (ASE) can degrade both upstream and downstream sensitivities. As a consequence, amplifier design has assumed increasing importance in bi-directional systems.
A variety of amplifier designs have been proposed for bi-directional optical communications systems. See, for example, J-M. Delavaux et al., "Repeated Bi-directional 10 Gb/s-240 km Fiber Transmission Experiment", Optical Fiber Technology 2, IEEE Photon Tech. Lett., pp. 1256-59 (1996); Shien-Kuei Liaw et al., "Amplified Multichannel Bi-directional Transmission Using a WDM MUX/DMUX Pair for Narrowband Filtering", Electronic Letters (Victoria, B.C., July 1997); Ken-ichi Suzuki et al., "Bi-directional Ten-Channel 2.5 Gbit/s WDM Transmission . . . ", OAA 97, PD 12-1 () and Chang-Hee Lee et al., "Bi-directional Transmission of 80 Gb/s . . . WDM Signal Over 100 km Dispersion", OECC 97 (Seoul Korea, 1997). While amplifier designs are well developed for single channel systems, designs for WDM systems continue to have a number of shortcomings. Accordingly there is a need for an improved amplifier for bi-directional WDM optical communications systems.